The arrival in 1987 of erbium-doped silica fibers has made it possible to develop high performance optical amplifiers capable of providing effective compensation for the losses suffered by light signals propagating along optical fibers in optical communications systems. Such erbium amplifiers are capable of replacing electronic repeaters, as have been used until now once every 50 km to 100 km for the purpose of regenerating (i.e. detecting and then re-emitting) optical pulses carrying digital information, thereby also eliminating the limitations on speed or on passband that have been imposed on optical communications by the in-line electronic components.
A particularly advantageous method of transmitting optical signals in systems that make use of optical amplifiers consists in transmitting solitons, which are optical pulses having pulse shape and intensity characteristics that are such that the two main sources of distortion in fibers, namely chromatic dispersion and non-linear refraction, cancel mutually. Solitons are therefore capable of propagating over thousands of kilometers without distortion of their shape, providing optical amplifiers are disposed periodically along the transmission line to inject energy into them for the purpose of compensating the attenuation losses they suffer during propagation.
Information corresponding to a sequence of "0" and "1" binary digits (known as "bits") can be encoded as a train of solitons by synchronizing the pulse rate with a clock which defines consecutive equal-time intervals (referred to as "windows" or as "bit-times") and by causing the state of the soliton occupying each bit-time to correspond to the binary value that is to be assigned thereto. The most commonly used kind of encoding consists in soliton on-off keying (OOK) in which the presence of a pulse during a bit-time corresponds to a "1" binary digit, whereas absence of a pulse corresponds to "0".
When signal attenuation is effectively compensated by in-line amplification (as is the case for trains of solitons), the reliability with which data is transmitted is generally limited by the presence of "noise", i.e. random fluctuations in the characteristics of the signal, e.g. its amplitude, its transmission speed, or its arrival time at the detector. In particular, fluctuations in the arrival time of solitons at the detector (known as soliton "jitter") can cause a soliton to be shifted into an adjacent bit-time, thereby giving rise to a data reception error. The reliability of a transmission system is characterized by its bit error rate (BER); in general, a BER of less than 10.sup.-9 is considered acceptable. For a soliton transmission line, jitter accumulation over the course of propagation implies that there exists some maximum propagation distance L.sub.max and some minimum bit-time (corresponding to a maximum soliton rate B.sub.max) beyond which the error rate rises to values that are unacceptable, given the specifications of the communications system. These two parameters are often combined into a single measure of the performance of the soliton transmission system; this measure or "figure of merit" is constituted by the product BL.
There are two main sources of inaccuracy contributing to soliton position in a frame of bit-times. The first source is due to the interactions that occur between adjacent solitons. When two solitons are propagating along the same fiber, they are subject to mutual attraction or repulsion: the sign and the magnitude of this "force" depend on relationships of phase, of distance, and of polarization between two solitons. This interaction can change the relative distance between two solitons and can cause them to be "pushed" into adjacent bit-times. Although this interaction is deterministic, and although its effects can, in some cases, be taken into account and corrected on reception, the quasi-random nature of a sequence of bits has the consequence of causing displacements due to interaction between solitons to have the appearance of random jitter, thereby giving rise to reception errors. In present soliton transmission systems extending over several thousands of kilometers, these interactions are kept to a relatively low level by imposing a gap between two consecutive solitons that is 5 or 6 times greater than the duration LS of the solitons themselves. For example, with solitons of 18 ps duration, it is necessary to have gaps (or bit-times) of 100 ps or 110 ps so as to ensure that interactions remain small over distances of several thousands of kilometers. This limits the bit rate of binary information as transported by solitons having LS=18 ps to B.sub.max =9 Gbit/s or 10 Gbit/s.
In this context, reference may advantageously be made to the following publication: [1] J. P. Gordon, "Interaction forces among solitons in optical fibers", Optics Letters, vol. 8, 596-598 (1983).
The second source of inaccuracy is noise of quantum origin that is injected during each amplification stage. This phenomenon has been described, in particular, in the following article: [2] J. P. Gordon and H. A. Haus, "Random walk of coherently amplified solitons in optical fiber transmission", Optics Letters, vol. 11, 665-667 (1986).
This noise consists in a random walk in the frequency of the soliton carrier wave, which, because of the chromatic dispersion of fibers, gives rise to random variation in soliton speed after each amplification stage, thus giving rise to "jitter", i.e. to random fluctuations in the arrival times of solitons at the detector.
Various techniques have recently been implemented to reduce soliton jitter and to increase transmission range, e.g. by introducing spectral filters or fast modulators at each amplification stage; proposals have also been made to use parametric amplification. Reference may advantageously be made to the various following publications: [3] A. Mecozzi, J. D. Moores, H. Haus, and Y. Lai, Optics Letters, vol. 16, 1841-1843 (1991); [4] Y. Kodama and A. Hasegawa, Optics Letters, vol. 17, 31-33 (1992); [5] L. F. Mollenauer, J. P. Gordon, and S. G. Evangelides, Optics Letters, vol. 17, 1575-1577 (1992); [6] M. Nakazawa, E. Yamada, H. Kubota, and K. Suzuki, Electronics Letters, vol. 27, 1270 (1991); [7] I. H. Deutsch and I. Abram, "Reduction of quantum noise in soliton propagation via phase sensitive amplification" Journal of the Optical Society of America (submitted for publication).
All of these techniques share two points in common: (1) they rely on in-line use of special components and (2) they are incapable of enabling soliton spacing to be reduced significantly below 6 LF.
An alternative technique which is based on compensating interactions between solitons in order to "stiffen" the train and impart a certain amount of jitter immunity thereto has also been proposed by Izo Abram in the European patent application filed under the number 93401616.3 and under the title "Procede de transmission optique a tres longue distance de solitons et systeme de transmission de mise en oeuvre de ce procede" [A soliton method of very long distance optical transmission, and a transmission system implementing the method.
That technique consists in using trains of equidistant solitons in which adjacent solitons present phase alternation of .pi. radians. That disposition ensures that the interactions between solitons are repulsive, thereby providing a "return force" whenever a soliton approaches one of its neighbors. This stabilizes the relative spacing of the solitons and fixes them in the bit-time frame.
In that proposal, encoding is performed by rotating soliton polarization through an angle, e.g. 45.degree., which preserves the repulsive nature of interactions between adjacent solitons to a very great extent.
Some of the advantages presented by that technique are the following: 1) It enables pulses to be moved closer together, to a spacing of 4 LS, which corresponds to an increase in bit rate while maintaining the other characteristics of the system constant; 2) no specialized equipment needs to be used in line other than the optical amplifiers that are already installed; and 3) it can be combined with one of the jitter-reduction techniques that are based on specialized equipment (e.g. the spectral filter technique), thereby enabling the advantages of two techniques to be combined.
Nevertheless, the technique of compensating interactions suffers from limitations when encoding is performed merely by rotating polarization, representing the digit "1" by rotation through 45.degree., while the digit "0" corresponds to the original polarization: adjacent solitons will have parallel polarization when they correspond to the same binary digit, whereas there will be an angle of 45.degree. between them when two solitons represent different binary digits. More precisely, and taking account of the phase alternation between adjacent solitons, these situations correspond to angles of 180.degree. and of 135.degree. respectively between the electrical field vectors of two successive solitons. These polarization relationships imply that the repulsive force between two solitons representing the same binary digit is greater than the repulsive force between two solitons corresponding to different binary digits. This causes the repulsive forces between solitons to be out of balance and, on reception, Gives rise to jitter appearing, thereby limiting the effectiveness of the technique of compensating interactions.